The present invention relates to a method of manufacturing an SOI substrate having a surface single-crystal silicon layer with a desired thickness.
Various structures have been conventionally proposed as semiconductor integrated circuits, and it is known that forming various devices in a silicon layer on an insulating layer is more advantageous than forming devices on a single-crystal silicon substrate in terms of device characteristics and isolation between devices; i.e., a parasitic capacitance can be reduced and devices can be completely isolated. From this viewpoint, methods of forming semiconductor integrated circuits on an SOI (Silicon On Insulator) substrate instead of a single-crystal silicon substrate become popular recently.
These SOI substrate manufacturing methods are roughly classified into a method of SIMOX (Separation by Implanted OXygen) and a method using wafer bonding.
The method of manufacturing a SIMOX substrate will be described first. In this method, a heavily doped oxygen layer is formed by implanting oxygen ions to a predetermined depth of a single-crystal silicon substrate. Annealing is then performed at a high temperature of about 1,300.degree. C. for a few hours to change the heavily doped oxygen layer into an electric insulating buried oxide layer. Subsequently, the oxide layer formed on the silicon substrate surface during the annealing is removed to form a buried oxide layer midway along the direction of thickness of the silicon substrate. The obtained SOI substrate has a single-crystal silicon layer with a predetermined thickness formed on the buried oxide layer.
The SOI substrate manufacturing method using wafer bonding will be described next. Note that the SOI substrate manufacturing method using wafer bonding includes two methods.
The first SOI substrate manufacturing method is as follows. Two single-crystal silicon substrates are prepared, and one of the silicon substrates is oxidized to form an oxide layer on the surface. The other silicon substrate is overlapped and bonded such that the oxide layer is sandwiched between the two substrates, thereby forming a structure consisting of an oxide layer, a first single-crystal silicon layer, an oxide layer (buried oxide layer), and a second single-crystal silicon layer (substrate silicon) in this order from the substrate surface.
Thereafter, the oxide layer is removed by polishing, and the first single-crystal silicon layer is polished to decrease its thickness, thereby forming a structure consisting of a surface single-crystal silicon layer, a buried oxide layer, and substrate silicon.
It is also possible to additionally perform AcuThin.sup.Th process (1993 IEEE SOI Conference Proc., 1993, pp. 66-67) after the polishing. If this is the case, a structure consisting of a surface single-crystal silicon layer, a buried oxide layer, and a substrate silicon layer in this order from the substrate surface is formed.
The second SOI substrate manufacturing method using wafer bonding will be described below. This manufacturing method does not use the polishing as described above (Japanese Patent Laid-Open No. 5-211128 and M. Bruel, Electronics Lett., 1995, Vol. 31, pp. 1201-1203).
In the first stage of this method, hydrogen ions or ions of a rare gas are implanted into an oxidized single-crystal silicon substrate to form fine bubbles in the substrate. In the second stage, this substrate is tightly adhered to another single-crystal silicon substrate. In the third stage, the adhered substrates are heat-treated to separate into two substrates from the bubble portion, forming a structure consisting of a surface single-crystal silicon layer, a buried oxide layer, and substrate silicon in this order from the substrate surface.
SOI substrates are manufactured as described above. The single-crystal silicon layer formed on the oxide layer has an effect on the characteristics of a semiconductor device such as an LSI including MOS transistors formed in this region. Therefore, it is necessary to accurately determine the thickness of the single-crystal silicon layer.
To accurately determine the thickness of the single-crystal silicon layer formed on the oxide layer, a method called sacrificial oxidation is proposed. In this sacrificial oxidation method, a surface single-crystal silicon layer having a thickness equal to a difference between a known surface single-crystal silicon layer thickness of an SOI substrate and a desired layer thickness in device design is changed into a thermal oxide layer by thermal oxidation, and then only this thermal oxide layer is removed. This sacrificial oxidation method is widely used since the method is superior in controllability and reproducibility.
Unfortunately, the use of the sacrificial oxidation method is unpreferable because of an increase in leakage current of a device formed in an SOI substrate, particularly leakage current between the source and drain of a MOS transistor.
This will be explained more specifically with reference to FIG. 9. FIG. 9 shows a structure having an n-type MOS transistor formed in a surface single-crystal silicon layer of a SIMOX substrate. Referring to FIG. 9, a buried oxide layer 2 is formed on substrate silicon 1, and a silicon semiconductor region having a source region 8, a drain region 9, and a body region 10 is formed on the buried oxide layer 2. This semiconductor region is surrounded by a device isolation region 3 consisting of such as a silicon oxide layer. A source electrode 16 is connected to the source region 8, and a drain electrode 17 is connected to the drain region 9. A gate electrode 6 is formed on the body region 10 via a gate silicon oxide layer 5, and a silicon oxide layer 7 and a PSG film 15 are formed on the gate electrode 6. In this structure, the source electrode 16 is grounded, the drain electrode 17 is connected to a positive power supply, and a positive bias is applied to the gate electrode 6.
An n-type MOS transistor with the above construction is fabricated as follows. A surface single-crystal silicon layer on a SIMOX substrate is changed into a thermal oxide layer to a depth of 132 nm from the surface by using the sacrificial oxidation method. Thereafter, this thermal oxide layer is removed, and transistors including an n-type MOS transistor are formed in the residual 50-nm thick surface single-crystal silicon layer. Note that the gate length of this MOS transistor formed in this example is 0.25 .mu.m, and the transistor is designed so that normally-off electrical characteristics are obtained.
It is known that a leakage current of an LSI device formed in an SOI substrate by the sacrificial oxidation method readily increases. For example, when the gate length of MOS transistors constituting an LSI device is about 0.5 .mu.m or less, a leakage current (to be referred to as an S/D leakage current hereinafter) particularly between the source and drain easily increases. Consequently, a standby current of the LSI device also increases.
FIGS. 10A and 10B show the drain current-drain voltage characteristics of an n-type MOS transistor set (a device in which about 20,000 MOS transistors were connected parallel to each other) according to FIG. 9 fabricated in a surface single-crystal silicon layer of a SIMOX substrate.
FIG. 10A shows the drain current-drain voltage characteristics when a large S/D leakage current was generated. FIG. 10B shows the drain current-drain voltage characteristics in a normal case. Note that sacrificial oxidation in each of FIGS. 10A and 10B was performed at 1,150.degree. C.
Comparing the characteristics shown in FIGS. 10A and 10B when gate voltage V.sub.G =0 (V) shows that a larger drain current than in FIG. 10B flows in FIG. 10A. That is, this type of the SIMOX substrate cannot be applied to a low-power LSI.